Microorganisms transform mercury (Hg) in ocean water to methylmercury (MeHg), which is very toxic. Because MeHg concentrates in living organisms, it eventually builds up to very dangerous concentrations in fish and higher animals in sea food webs, damaging both human and ecosystem health. However, we do not know exactly how the methylation occurs, which microorganisms do it and especially which fraction of Hg is available to them. We call this part the bioavailable fraction of Hg, it represents a small percentage of total Hg and its chemical identity is largely unknown. Bioavailable Hg in water can be elemental or ionic Hg species, associated with a great variety of other particles or dissolved organic in water column. Hg has also seven stable isotopes and their distribution in Hg species is very informative about the processes.
We will focus on this bioavailable fraction of Hg, the strength with which it is bonded, the processes that control its abundance and transformations, and the likelihood that Hg gets methylated. This will tell us in what conditions methylation happens so that we can in the future discover microbes that do it. To that end, we will be the first to measure Hg stable isotopes of bioavailable fraction,
collected with Diffusive Gradient in Thin-films samplers onboard an automatic submarine in Adriatic Sea. With this extremely sensitive method we can describe conditions that support Hg methylation and predict MeHg stress in ecosystems.